Strong upward trend

Climate change has significantly increased heavy rainfall

(Bild: juliasudnitskaya)

Compared to 1980, short-term heavy rainfall events in Austria today bring significantly more precipitation. This is shown by an analysis of two 100-year measurement series presented by experts in the scientific journal "Nature". There is a "strong upward trend", especially for large, rapidly occurring amounts of precipitation.

Hydrologist Günter Blöschl explains: "On average, rainfall now brings 15 percent more water. The reason is higher temperatures.

It is not every day that a study with data coming entirely from Austria, evaluated by researchers from the Vienna University of Technology (TU), Geosphere Austria, the Ministry of Agriculture and the University of Graz, makes it into the journal "Nature". However, the findings are valid for large parts of the world and especially for the mid-latitudes, explained Blöschl, showing how the increased temperatures affect the regional level and what this means for the flood situation.

Günter Blöschl from the TU Vienna

(Bild: Franz Pfluegl)

Two series of measurements span over 100 years
As precipitation in Austria has been meticulously documented in parallel by two independent bodies over 100 years - firstly by Geosphere Austria, the former Central Institute for Meteorology and Geodynamics (ZAMG), and secondly by the Hydrographic Service - this can be examined in detail here. There are a total of 883 measuring stations nationwide, 163 of which also collect hourly data. According to the hydrologist, the scientists were able to analyze precise measurement series from 1900 to 2023, which is unprecedented.

One of the big questions in climate research and for flood management is how rising average temperatures due to climate change - in Austria the increase is currently around two degrees Celsius - will affect heavy rainfall events, especially in the summer months. This is based on the knowledge that air can absorb seven percent more water vapor for every additional degree Celsius. Questions also arise as to how the additional energy affects the regional system. The stronger warming near the ground also means that the warm air masses rise more quickly from there. This means that they cool down more quickly further up, which leads to more rain, explained Blöschl.

Flooding is to be expected more often in the future - as was the case in Lower Austria last year.

(Bild: HELMUT FOHRINGER)

Average heavy rainfall today much heavier
Accordingly, it was assumed that the short-term precipitation events would have to intensify. However, according to the hydrologist, this is the first time that it has been possible to determine this precisely at this level. In Austria, the data even provides an evaluation of hourly rainfall from 1950 onwards. In fact, there was no significant change from 1950 to 1980. However, the trend then continued upwards for more than 40 years: "An average heavy rainfall in the period 2003 to 2023 brought 15 percent more water than an average heavy rainfall in the period 1950 to 1970," says lead author Klaus Haslinger from Geosphere Austria in a press release.

This development has less to do with the large global circulations in the atmosphere, but is largely explained by the additional energy that is available locally, says Blöschl: "This also means that this happens more or less everywhere outside the tropics in a similar way. This is the first study worldwide to show this."

Smaller increase in extreme daily amounts
The fact that it is mainly short-term heavy rainfall events that are more severe is shown by the fact that the amount of rainfall within an entire day during extreme events increased by just eight percent from the 1990s to the 2010s. So if short-term precipitation events are increasing more than longer periods of intense rainfall, this also means that different regions are affected differently by flooding.

According to the study results, flash floods caused by heavy thunderstorms lasting one or two hours and floods in smaller river catchment areas can therefore be expected even more in the future. The fact that this is the case can already be seen in the domestic flooding data. "However, this does not produce floods in the Danube," said Blöschl.

What happens regionally in the event of flash floods?
This means that more thought needs to be given to regional flood protection in smaller areas and a precise analysis of the effects of even more intense, short heavy rainfall events in a specific region. According to the hydrologist, the intensity of short periods of heavy rainfall is increasing on average throughout Germany as a result of global warming.